Steam tube dryers, for example, are known as conventional indirect-heating rotary dryers. Such a steam tube dryer heats and dries a substance to be dried by a plurality of heating pipes installed in a revolving cylinder, into which a heating medium such as steam is introduced. These steam tube dryers are superior in drying potential because the heating area per capacity is large. They are also characteristic in high heat transfer rate and also superior in easiness of operation, and thus have been used for drying coal for coke furnace and chemical products.
The basic structure for these steam tube dryers is shown in FIG. 2. A substance to be dried such as wet or granular powder is brought into contact with heating pipes 311 heated by a heating medium in a revolving cylinder 310 and sequentially fed to and discharged from an outlet port 322 continuously by revolution of the revolving cylinder 310. A heating medium such as heated steam is introduced then into heating pipes 311 through a heating medium inlet pipe 361 installed on a revolution joint 360, fed through each heating pipe 311, and discharged through a heating medium outlet pipe 362. The evaporant from the substance to be dried is discharged, as it is carried in the carrier gas G supplied from an inlet port 341 located to the discharge side of the dried substance, out of the revolving cylinder 310 through an outlet port 342 on the inlet side of the substance to be dried (Patent Document 1).
Traditionally known methods of producing a solid fuel from a raw porous coal include, for example, the method of producing a solid fuel described in Patent Document 2. The method will be described briefly with reference to FIG. 9. A porous coal (raw coal) is pulverized in a pulverization step and mixed with a mixed oil containing a heavy oil fraction and a solvent oil fraction in the mixing step, to give a raw slurry. The raw slurry is then preheated and additionally heated in a vaporization step for further progress of dehydration of the porous coal and impregnation of the mixed oil into the micropores of the porous coal, to give a dehydrated slurry. Then in a solid-liquid separation step, the modified porous coal and the mixed oil are separated from the dehydrated slurry, and the modified porous coal is dried in the final drying step. The dried modified porous coal is cooled and molded as needed, to give a solid fuel. On the other hand, the mixed oil recovered in the solid-liquid separation step and the final drying step is circulated and fed to the mixing step of preparing a raw slurry, for recycle as a circulation oil.
Generally in the final drying step of the method above, the modified porous coal separated in the solid-liquid separation step is dried as it is heated and conveyed in an indirect-heating rotary dryer and a carrier gas is fed thereto. The indirect-heating rotary dryer known is, for example, a so-called steam tube dryer. Specifically, as shown in FIG. 10, a slurry S of the substance to be dried is separated into solid and liquid in a centrifugal separator 101 and the solid is fed into an indirect-heating rotary dryer from a first side (left side in FIG. 10) of a revolving cylinder 105 and the dried substance is discharged out of a discharge chute 106 from a second side (right side in FIG. 10). The carrier gas is then supplied from a particular direction opposite to the conveying direction of the modified porous coal in the dryer, from the viewpoint of drying efficiency (oil recovery efficiency) (see, for example, Patent Document 3). The discharged carrier gas is fed into a wet scrubber 111 for recovery of the fine dust simultaneously conveyed and subjected to spray dust collection, while the liquid containing the liquid separated in centrifugal separator 101 is circulated to the wet scrubber 111, and the dust is captured by spray cooling by using the circulating liquid in the higher region. The solution recovered from the recovery unit is stored temporarily in a storage tank 113, then cooled in a condenser 114, and used for spray cooling.    Patent Document 1: JP-A No. 2005-16898    Patent Document 2: JP-A No. 7-233383    Patent Document 3: JP-A No. 61-250097